1. Field of the Invention
Various embodiments of the invention described herein relate to the field of capacitive sensing input devices generally, and more specifically to a switchable charge acquisition circuit and accompanying circuitry for sensing mutual capacitances associated with a touchscreen.
2. Description of the Prior Art
Two principal capacitive sensing and measurement technologies are currently employed in most touchpad and touchscreen devices. The first such technology is that of self-capacitance. Many devices manufactured by SYNAPTICS™ employ self-capacitance measurement techniques, as do integrated circuit (IC) devices such as the CYPRESS PSOC™. Self-capacitance involves measuring the self-capacitance of a series of electrode pads using techniques such as those described in U.S. Pat. No. 5,543,588 to Bisset et al. entitled “Touch Pad Driven Handheld Computing Device” dated Aug. 6, 1996.
Self-capacitance may be measured through the detection of the amount of charge accumulated on an object held at a given voltage (Q=CV). Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, additional charge is attracted to the electrode. As a result, the self-capacitance of the electrode increases. Many touch sensors are configured such that the grounded object is a finger. The human body is essentially a capacitor to a surface where the electric field vanishes, and typically has a capacitance of around 100 pF.
Electrodes in self-capacitance touchpads are typically arranged in rows and columns. By scanning first rows and then columns the locations of individual mutual capacitance changes induced by the presence of a finger, for example, can be determined. To effect accurate multi-touch measurements in a touchpad, however, it may be required that several finger touches be measured simultaneously. In such a case, row and column techniques for self-capacitance measurement can lead to inconclusive results.
One way in which the number of electrodes can be reduced in a self-capacitance system is by interleaving the electrodes in a saw-tooth pattern. Such interleaving creates a larger region where a finger is sensed by a limited number of adjacent electrodes allowing better interpolation, and therefore fewer electrodes. Such patterns can be particularly effective in one dimensional sensors, such as those employed in IPOD click-wheels. See, for example, U.S. Pat. No. 6,879,930 to Sinclair et al. entitled Capacitance touch slider dated Apr. 12, 2005.
The second primary capacitive sensing and measurement technology employed in touchpad and touchscreen devices is that of mutual capacitance, where measurements are performed using a crossed grid of electrodes. See, for example, U.S. Pat. No. 5,861,875 to Gerpheide entitled “Methods and Apparatus for Data Input” dated Jan. 19, 1999. Mutual capacitance technology is employed in touchpad devices manufactured by CIRQUE™. In mutual capacitance measurement, capacitance is measured between two conductors, as opposed to a self-capacitance measurement in which the capacitance of a single conductor is measured, and which may be affected by other objects in proximity thereto.
Simultaneously driving of all drive electrodes or lines on a touchscreen can increase the dynamic range signals appearing on the sense electrodes or lines, and presented to the corresponding sense circuitry, according to the number of drive electrodes driven at one time. Handling the resulting increased dynamic range of charge signals presented to the sense circuitry can be accomplished by using conventional charge integrator readout circuits having increased feedback capacitor values. As alluded to above, such feedback capacitor values are increased according to the number of simultaneously driven drive electrodes, which in a large touchscreen may require an increase of feedback capacitance by a factor of 20 or more. If high drive voltages are used to increase touch signal noise immunity in a touchscreen, feedback capacitor values in charge integrator circuits incorporated into the sensing circuitry must also typically be increased. Large feedback capacitor values present certain well-known problems when actually implemented in a touchscreen system, however, such as an increased amount of area being required in an integrated circuit implementation. While active current division circuits preceding charge integrator circuits may be employed to reduce feedback capacitor size, doing so requires the use of additional amplifiers and resistors, which are less operationally and temperature stable components compared to capacitors.
What is needed is a capacitive measurement or sensing circuit or system that may be employed in touchscreen and touchpad applications that does not require the use of large feedback capacitors or the use of active current division circuits.